The invention relates to a chain tensioner or tightener with a flat, curved bow or rail made from a highly deformable plastics material and acting as a sliding shoe or contact shoe, one end of the bow being fixed in rotary manner to a carrier, whilst its other end displaceably engages on the carrier and is resiliently pressed against the chain.
A chain tensioner with these features is known (German Pat. No. 1 750 684) and serves to prevent the vibration of a chain or a V-belt, the chain or V-belt being simultaneously tensioned with the given force or tension. Such sliding shoes are preferably used as slide rails for the drive of camshafts in motor vehicle engines. This construction made from injection moulded plastic suffers from disadvantages with respect to its operating behaviour and manufacture, whilst in the long term it is not able to withstand the stresses which occur. Such a plastic bow in fact undergoes position changes as a result of cold flow or creep. It is therefore necessary to work with corresponding wall thicknesses, so that such a bow is able to withstand the mechanical and thermal stresses which occur. However, the use of greater wall thicknesses is accompanied by the risk of shrinkage cavity formation. As a result of the dimensional changes of the sliding shoe caused by the stresses which occur, there is also a modification to the fixed position and consequently an uncontrolled chain slip.
In another known construction, the bow has a part cast from metal and onto it is then engaged a separately manufactured plastic belt. However, this construction also suffers from serious disadvantages, the plastics part deforming differently from the metallic support part, so that the two parts can become detached from one another. There is a tendency of the plastics part to flow away in the case of fatigue stresses due to cold flow or creep. Due to these stresses, shape changes occur, which have an effect on the chain curve, which consequently is not given the constructionally established position. A change takes place to the tangential entry on the sliding shoe and consequently there is a change to the angle between the entry of the chain tensioner and the chain or, in other words, the chain meets the chain tensioner at an angle differing from that originally intended.